This invention relates to drivelines for hybrid motor vehicles and in particular for motor vehicles such as all terrain vehicles which have selective two wheel drive or four wheel drive.
A hybrid motor vehicle is one which is equipped with a primary mover, such as an internal combustion engine and a secondary mover, such as an electric motor.
There are various known ways in which the primary and secondary movers can be arranged in order to drive the vehicle and they may be arranged to drive either one axle or two.
EP0584090 discloses a hybrid vehicle having permanent four wheel drive. The vehicle is provided with an internal combustion engine and an electric motor. The electric motor is combined with the rear differential and can provide drive to the front wheels via a drive shaft and to the rear wheels via the rear differential.
It is also known to configure a hybrid vehicle so that it may be selectively driven either by the engine or the electric motor or by both power sources simultaneously. One or more clutch arrangements may be used.
U.S. Pat. No. 6,308,794 discloses a hybrid drivetrain which utilises two clutches. A first clutch is provided between the engine and the transmission. A second clutch is integrated with the electric motor and allows for disengagement of the motor from the transmission during operation of the engine.
Some conventional (i.e. not hybrid) all terrain vehicles which have a four wheel drive system are provided with a means of disconnecting two of the driving wheels when the vehicle is driven on less challenging terrain e.g. “on road”. An example of such a four wheel drive system is shown in U.S. Pat. No. 5,105,901 in which the drive to the rear wheels may be broken through a clutch device to establish two wheel drive when four wheel drive is not necessary. The drive to the rear wheels is also controlled by wheel clutches which are utilised for controlling the torque distribution between the right and left rear wheels to improve controllability when cornering.
During two wheel drive conditions, the driveline components connected to the rear wheels may be caused to rotate due to frictional drag between the components in the driveline. This driveline drag is a major contributor to fuel inefficiencies and ways have been sought to minimise the residual drag in four wheel drive systems operating in two wheel drive mode. One method of reducing driveline drag is to disconnect the main rear wheel drive shaft (propshaft) from the rear wheel rear drive shafts by using a differential of the type sold by GKN under the name of the TFS free running differential. This differential allows the rear drive shafts to be disconnected from the differential casing so that there are no losses between the differential bevel gears and the pinions and no drive to the main drive shaft. Such systems are generally switched between four wheel drive mode and two wheel drive mode when the vehicle is stationary.
The present invention seeks to utilise a hybrid vehicle configuration to provide a four wheel drive system in which the reconnection between two wheel drive mode and four wheel drive mode may be made in a smooth manner while the vehicle is being driven. This may be termed “dynamic driveline reconnect”. However, a problem with dynamic driveline reconnection is that the reconnection may be felt by the driver as an unacceptable “jerk”, i.e. a spasmodic acceleration.
One known arrangement for minimising the jerk felt in a conventional vehicle when the reconnection is made is disclosed in GB-A-2407804. In one embodiment of this known arrangement, the propshaft is decoupled from the engine and the rear wheels by means of clutch arrangements at each of its ends. A reasonably smooth transition from two wheel drive to four wheel drive is achieved by firstly re-engaging one of the two clutches, allowing the propshaft to spin up to the rotational speed of the engine (or rear wheels) and then re-engaging the other clutch. A problem with this known arrangement is that a jerk may still be felt when the first clutch re-engages. Further, there is a delay in transferring from two wheel drive to four wheel drive while the partially reconnected driveline is spinning up.